A ring resonator can be regarded as a ring-shaped waveguide. The resonance wavelength λ of the mth mode in a ring resonator is λ=2πrneff/m, where r is the resonator radius, neff is the effective refractive index (RI) of the mth mode of the ring resonator, and m is an integer. Light propagating in a ring resonator circulates along the ring resonator and has an evanescent field that can reach several hundred nanometers into the surrounding medium (e.g., liquid, gas, and polymer coatings, etc.), which can contain analytes. Therefore, the light propagating through the ring resonator can interact repeatedly with the analytes on or near the surface of the ring resonator.
When used as sensors, ring resonators can have a long interaction length with small physical sizes. For conventional linear waveguide sensors or fiber sensors, the interaction length between light and analytes is essentially the physical length of the waveguide/fiber. In comparison, ring resonator sensors can create an extremely long effective interaction length by circulating the light multiple times in the resonator. The effective interaction length can be expressed as Leff=Qλ/(2πn), where Q is the quality factor, or Q-factor, of the resonator. Depending on ring resonator configuration, the Q-factor can be about 104 to about 108. Therefore, even with a small physical size (e.g., on the order of microns), the ring resonator sensor can have an effective interaction length of a few tens of centimeters or even longer, resulting in higher sensitivity, smaller footprint, and higher multiplexing capability while using less analyte.
Ring resonator sensors can be used for detecting a variety of chemical and biological species including, but are not limited to, DNA, proteins, viruses, nanoparticles, bacteria, heavy metals, pesticides, and volatile organic compounds (VOCs) in liquid or gaseous phases.